Trees offer real world benefits of carbon reduction, temperature reduction, shade for people, the psychological benefits that trees offer, some limited wildlife habitat, and they do it without much outside help. They grow themselves with decent maintenance.
But you have to build and maintain this tank. What carbon was used to do so, and what maintenance will it need. Can it offset its own cost? It offers no benefits to wildlife, no shade, no temperature reduction.
Yeah, trees leave leaf litter and can heave sidewalks with roots, but given that neither system is perfect, there’s no reason to argue that boxes of algae are better.